The present invention relates to an electrophotographic photosensitive body and electrophotographic apparatus and, more particularly, to an electrophotographic photosensitive body and electrophotographic apparatus used in a wet electrophotographic technology.
In an electrophotographic technology, the transfer efficiency of toner is an important factor that determines the image quality. For example, if a transfer efficiency of 100% is not realized, i.e., toner is not completely transferred onto paper, the degradation in image quality appears as a decrease in image density or an image blur. If the transfer efficiency from a photosensitive body such as a photosensitive drum to paper or an intermediate transfer roller is low, the amount of toner remaining on the image holding surface of the photosensitive body after transfer is large, and a more powerful cleaner must be used. As a result, the image holding surface of the photosensitive body is considerably damaged by the cleaner, resulting in a shorter life time of the photosensitive body. Hence, a demand for realizing a sufficiently high transfer efficiency, i.e., a transfer efficiency of almost 100% has arisen.
As electrophotography, currently, dry electrophotography is popular. In dry electrophotography, as a developer, a single component developer made of toner particles containing ferrite or a dual component developer made of a mixture of carrier particles of ferrite and toner particles is used. That is, this technology uses a developing powder.
Since such a developing powder is used in dry electrophotography, the problem associated with the transfer efficiency is relatively simple. For example, in this technology, a sufficient transfer efficiency has already been realized by improving the transfer process or decreasing the adhesion between the image holding surface of the photosensitive body and the developing powder.
To the contrary, in wet electrophotography, a liquid developer formed by dispersing toner particles into a petroleum solvent is used as a developer. For this reason, as will be described below, the problem associated with the transfer efficiency in wet electrophotography is much more complex than that in dry electrophotography.
In wet electrophotography, toner having a particle diameter on the submicron order is used, which is much smaller than the particle diameter of toner used in dry electrophotography. For this reason, wet electrophotography is expected to realize a higher image quality. In addition, since a sufficient image density can be obtained in a small amount of toner, wet electrophotography is expected to be economical and realize the texture of printing (e.g., offset printing). Furthermore, since toner can be fixed to paper at a relatively low temperature, wet electrophotography is expected to realize energy saving.
However, when toner having such small particle diameter is dispersed into a petroleum solvent, although the above-described advantages are expected, the adhesion between the image holding surface of the photosensitive body and the developer eminently increases. For this reason, if a developer image is to be transferred by an electric field transfer scheme, i.e., using electrophoresis of toner particles in the petroleum solvent, a large potential difference must be applied between the photosensitive body and the transfer roller and the like. However, normally, such large potential difference can not be applied. Hence, in wet electrophotography, a sufficient transfer efficiency can hardly be realized using only the electric field transfer scheme.
In wet electrophotography, not the electric field transfer scheme but the offset transfer scheme using heat or pressure to transfer a developer image can be used. When this scheme is employed, a high transfer efficiency can be realized by coating the image holding surface of the photosensitive body with a silicone resin or fluororesin. However, when a silicone resin or fluororesin is only simply applied to the image holding surface of the photosensitive body, an image omission, i.e., partial omission of the developer image on the image holding surface, or a so-called image blur in which the toner forming the developer image extends outward from the electrostatic latent image may occur. If the developer image is distorted, no satisfactory image quality can be obtained anymore.
The image blur readily occurs when the amount of solvent remaining on the image holding surface is large. To prevent this, an examination has been made to remove the excess solvent from the image holding surface of the photosensitive body before transfer, but the obtained effect is still insufficient.
As described above, in wet electrophotography, when the image holding surface of the photosensitive body is modified to improve the transfer efficiency, an image omission or an image blur must be taken into consideration. Unlike dry electrophotography, the adhesion between the image holding surface of the photosensitive body and the developer is influenced by not only the interaction between the powder and the image holding surface of the photosensitive body but also the interaction between the solvent and the image holding surface of the photosensitive body.
More specifically, since the developer used in wet electrophotography contains the solvent, the influence of the solvent on the adhesion between the image holding surface of the photosensitive body and the developer must be taken into consideration. In addition, an image omission or image blur that need not be taken into consideration in dry electrophotography must also be taken into consideration. For these reasons, wet electrophotography can hardly realize a satisfactory image quality.
It is an object of the present invention to provide an electrophotographic photosensitive body and wet electrophotographic apparatus, which can realize a satisfactory image quality in wet electrophotography.
It is another object of the present invention to provide an electrophotographic photosensitive body and wet electrophotographic apparatus, which can realize a sufficiently high transfer efficiency in wet electrophotography without causing any image omission or image blur.
According to the first aspect of the present invention, there is provided an electrophotographic photosensitive body used in a wet electrophotographic apparatus, comprising a substrate having a conductive surface, and a photosensitive layer formed on the conductive surface of the substrate and changing in a charged state upon being irradiated with light, the photosensitive layer having, on an exposed surface, Cxe2x80x94F bonds and Cxe2x80x94H bonds for which the ratio of the number NCF of Cxe2x80x94F bonds to the number NCH of Cxe2x80x94H bonds satisfies, on the exposed surface of the photosensitive layer, a relationship represented by
2/100 less than NCF/NCH less than 300/100.
According to the second aspect of the present invention, there is provided a wet electrophotographic apparatus comprising an electrophotographic photosensitive body having an image holding surface comprising a substrate having a conductive surface, and a photosensitive layer formed on the conductive surface of the substrate and changing in a charged state upon being irradiated with light, an exposed surface of the photosensitive layer constituting the image holding surface, Cxe2x80x94F bonds and Cxe2x80x94H bonds existing on the image holding surface for which the ratio of the number NCF of Cxe2x80x94F bonds to the number NCH of Cxe2x80x94H bonds satisfies, on the image holding surface, a relationship represented by
2/100 less than NCF/NCH less than 300/100
a latent image formation unit forming a latent image on the image holding surface, a developing unit forming a developer image on the image holding surface having the latent image using a liquid developer, and a transfer unit transferring the developer image from the image holding surface onto a recording medium.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter